Construction of bimetallic articles



y 31, 1955 c. E. STEVENS, JR 2,756,488

CONSTRUCTION OF BIMETTALLIC ARTICLES Filed Sept. 15, 1949 I I i I l I I I l I I m N (QBHDNI) sszmnoml 4 IN ENTOR FIG. I.

BY /%L W ATQR/NEYS CHARLES E. STEVENSJR.

United States Patent CONSTRUCTION OF BIIVIETALLIC ARTICLES Charles E. Stevens, Jr., Garden City, N. Y., ass'ignor to Fairchild Engine and Airplane Corporation, Farmingdale, N. Y., a corporation of Maryland Application September 15, 1949, Serial No. 115,795

3 Claims. (Cl. 29-1835) This invention relates to the manufacture of bimetallic articles, and has particular reference to the manufacture of bimetallic articles which are subjected to such wide temperature fluctuations as are likely to impair the union between the two metals.

Bimetallic articles of the aforementioned class are composed of a member having the property of strength and a member bonded thereto having the property of abstracting and dissipating heat from the strength member. Metals having such dissimilar properties usually are dissimilar in other less advantageous respects "such as having widely different coefficients of thermal expansion. Common examples of such metals are the ferrous .metals and aluminum and aluminum base alloys, which have been bonded together according to the process disclosed in Patent No. 2,396,730 and many articles subject in use to rigorous mechanical and thermal treatment have been produced successfully thereby, such as bimetallic internal combustion engine cylinders and pistons, pump cylinders and pistons, bearings, vacuum tube casings, brake drums, gears, and the like.

The thin film of ferro-aluminum alloy formed in that process at the interface between the relatively thick ferrous and aluminum members very strongly resists .separation between the members even when the article is subjected to rigorous physical and thermal treatment, notwithstanding that the ferro-aluminum alloy constituting the bond material is in itself not 'very strong, being quite brittle. Nevertheless, any failure or other impairment in the article resulting from extraordinarily rigorous treatment usually occurs in one of the members, due either to its design or in its inherent physical properties. In some cases, where the article is tubular or cylindrical and is subjected to very wide temperature fluctuations with sustained but irregular periods of high temperature and at the same time to heavy physical loads, the widely different coefiicients of thermal expansion of the aluminum and the most common of the ferrous metals, steel, impose a very great strain on the bond, tending to rupture the brittle alloy constituting it. Although this condition can be minimized by alloying truth the aluminum, a metal which will decrease the thermal expansion of the aluminum, that expedient results in impairment of the desirable heat transmission and dissipating property of the aluminum which caused it to be chosen in the first instance.

An example of such a cylindrical bimetallic article subjected to wide temperature fluctuations and concurrent heavy loads is a brake drum, comprising a cylindrical inner friction member of ferrous metal like steel, adapted to be engaged by the brake shoes, and a thick outer muif or blanket of aluminum bonded to the outer surface of '-the steel cylinder and having fins or ribs for more rapidly dissipating the friction heat abstracted by the aluminum from the steel friction member. Because of the widely different thermal expansion coeflicients of these united tubular members, the outer aluminum member is normally stressed in tension, the inner steel memher is normally stressed in compression and the bond is normally stressed in compression.

It has been found that when the article is heated above a certain temperature, these stresses are reversed, so that the outer aluminum member becomes stressed in compression, the inner ferrous member becomes stressed in tension, and the bond becomes stressed in tension. As long as the tension stress on the bond is substantially uniform per unit -of its area, the bond is not dangerously strained, and remains unimpaired, even when the temperature rises to a high degree to increase the tension in the bond, a condition which occurs in internal combustion engine cylinders, for example.

However, when a rapid rise to a high degree of temperature is accompanied by the simultaneous imposition of irregular physical loads which may not result in a uniform strain on the bond, as when a heavily-loaded vehicle like a bus or truck having bimetallic steelaluminum brake drums, is braked down hill continuously but non-uniformly, thermal stress conditions in the bimetallic brake structure are not only reversed, but the non-uniform physical load added to the thermal tension in the bond may cause the brittle alloy constituting the bond material to rupture or fracture. This tendency is more pronounced in a brake drum, for example, ina'smuch as the heavy, dragging load of the brake shoes creates a circular slippage shear in the bond between the members which might result in ultimate physical failure of a bond already weakened for the reasons described. Accordingly, a bimetallic structure having the extraordinary strength to withstand without impairment extremely rigorous thermal and physical treatment, Whether such treatment is expected or unexpected for the structure, would be highly desirable.

In accordance with the present invention, a bimetallic article formed of two members bonded together and composed of metals having difierent coefiicients of thermal expansion is provided, together with the method of making the same, which may be subjected to very wide temperature fluctuations and concurrent physical loads without impairment of the bond uniting the two members.

The invention comprises making bimetallic structures of two members bonded together and formed of metals having dissimilar expansion coefficients, such as steel and aluminum, the non-ferrous high expansion member having thickness which is predetermined according to an empirical formula such that the relative tension and compression stresses that obtain in the outer and inner members and in the bond at and during a normal tempera- 'ture range of usage are maintained at temperatures above that range, whereby the bond is not impaired and since the bond is not impaired, the strength and heat conductivity and other properties of the bimetallic article are retained even with cylindrical articles subjected to the most rigorous physical and thermal operation conditions.

For a more complete understanding of the invention, reference may be had to the accompanying drawings, in which:

Figure 1 illustrates by way of example, a transverse section of a brake drum made according to and embodying the invention, and

Fig. 2 is a graph illustrating the necessary aluminum blanket or mutt thicknesses provided by this invention for different temperatures.

Referring to Fig. 1, illustrating a brake drum as one example of a bimetallic article embodying this invention, numeral 10 designates the cylindrical steel drum whose internal circular surface is adapted to be frictionally engaged by the brake shoes as the drum rotates with the corresponding wheel, not shown. Formed on the outer surface of the drum 10 and bonded therewith, preferably by the process disclosed in said Patent No. 2,396,730, is a relatively thick muff or blanket 11, of high heat conductivity metal such as aluminum or aluminum base alloy. The ferro-aluminum bond film uniting the members and 11 is designated 12. The member 11 may have heat radiating ribs or fins 13 formed in casting the member '11 or milled therein after casting, as required.

In the illustrative case, Where the outer cylindrical blanket or muff 11 is formed of aluminum or aluminum base alloy having a high coefiicient of thermal expansion in addition to the desired property of high heat conductivity, the outer high expanding member 11 will shrink more as it cools from casting or solidification temperatures than will the inner low expanding ferrous member 10, notwithstanding that both were heated to the same temperature. When fully cooled, the stress conditions in the bimetallic article 10, 11, 12 will be such that the outer or non-ferrous member 11 will be stressed in tension, the inner or ferrous member will be stressed in compression, and the bond 12 homogeneously uniting the two members 10 and 11 will be stressed in compression.

Upon being placed in service, a brake drum is often called upon to dissipate quantities of heat of such magnitude that the temperature of the brake drum may rise to the range of 600 F. to 1100 F. or even beyond. As the above-described bimetallic article is heated, a condition of stress reversal in all components is reached at some temperature Tr. This is due to the fact that the greater coefiicient of thermal expansion of the non-ferrous member 11 causes it to expand more than the ferrous member 10. At the temperature Tr, the expansion is such that, were it not for the bond 12, the outer member would be a line-to-line fit on the inner member 10 and there exists a neutral stress condition, i. e., there is no stress in any of the three components. When heating is continued above this temperature Tr, the bond 12 alone keeps the two other members 10 and 11 in contact and the stress condition is now such that the outer or non-ferrous member 11 is stressed in compression, the inner or ferrous member, 10 is stressed in tension, and the bond 12 homogeneously connecting the two, is stressed in tension.

It has been found that Tr for bimetallic assemblies composed of an outer heat dissipating volume of aluminum and aluminum alloys is in the range of 250 F. to 400 F., the variation being accounted for by differences of aluminum alloy, and thickness and shape of the aluminum volume. For example, for a good bimetallic brake drum design, Tr will have a value of about 270 F.

Inasmuch as Tr is relatively low, a temperature rise to adegree materially above the temperature neutral stress temperature Tr tends to aggravate the stress conditions, since aluminum and its base alloys continue to expand as the temperature rises, thereby increasing the tension stress on the bond 12. Hence, the design of the outer or nonferrous member 11 must be constrained within certain limits or the tension in the bond 12 will rise to such a magnitude that the bond 12 will fracture in tension, resulting in great loss in heat conducting potential, and possible physical separation, particularly where the tension in the bond 12 is accompanied by physical load rendering the tension on the bond non-uniform per unit of area. Accordingly, the design of the heat dissipating non-ferrous volume 11 must be of such cross-section that it is weaker in compression at any operating temperature T than the bond 12 connecting the two metal or alloy members 10, 11.

The factor governing the design of a bimetallic article is determined by calculation, or test, or both, and depends upon the compression strength at any operating temperature of the metal selected for its advantageous properties, such as the high thermal conductivity of aluminum, its antifriction properties where the high conductivity metal is a bearing surface, and the like. For example, this design factor, hereinafter called K, will have a value of about 1.165 X 10 for an average brake drum installation and cooling conditions.

On the basis of these determinations, it has been ascertained that in order to maintain the bond 12 under compression during temperature rises up to, say 1100" F., and even higher, so that the bond will not be impaired even when simultaneously subjected to non-uniform heavy shear and other loads, the non-ferrous blanket or muff 11 must not exceed the thickness defined by the following formula:

25 eotan B wherein t=thickness of the aluminum muff or blanket 11 as indicated in Fig. 1; K=the aforementioned design factor; oc=th6 coefficient of thermal expansion of the nonferrous metal constituting the muff or blanket 11; and

T TT where T is the maximum operating temperature to which the article is likely to be subjected and Tr is the neutral stress-reversal temperature of the article having an indiscriminate thickness for 11.

By way of application of the principle of this invention to a brake drum mufi' or blanket of commercially pure aluminum, for example, and applying to the-formula the aforementioned values of K=l.165 10 T=1100 F., Tr=270 F., (2:.00001234, then t X cotan cotan .7545:.000944X75.94=.07 17 inch Fig. 2 illustrates the curve indicating the maximum thickness of mutt or blanket 11 for various temperatures up to 1000" F., as calculated according to the aforementioned formula. Above 1000 F. the curve is asymptotic, up to the limit of its solid state, which, for commercially pure aluminum, is about 1215 F. If an article is required to operate at a temperature near the limit of its being in the solid state,,cotan ,8 may be taken as unity and the formula becomes simplified to 'It will be observed that the most critical temperature range lies between about 300 F. and about 600 R, which is at least in part the operating rang of the majority of bimetallic articles contemplated herein. It will also be observed that the curve is asymptotic below 300 F., and that the maximum thickness of the muff or blanket 11' below that temperature makes virtually little or no difference.

By relying on curves like that of Fig. 2 for ascertaining the maximum thickness of various metals and alloys constituting the muff or blanket 11, one may be assured of a strong bond 12 under allknown operating conditions, inasmuch as the bond is under compression at all times, even though the stress conditions would otherwise reverse above Tr in an article of indiscriminate thickness of mufi or blanket 11. Even where the bimetallic article is not expected to be subjected to temperatures higher than the stress reversal point Tr, it is desirable .to conform its muff dimensions to those prescribed by this invention so as to guard against impairment of the bond in the event that the article "is inadvertently subjected to unexpected conditions. Thus, pump as well as engine pistons and cylinders should utilize the invention,'and hearings or bushings likewise, in case of overheating due to lubrication failure.

Although the disclosure of the method and article of this invention has'been directed to a bimetallic brake drum, it is to be understood that the invention is not limited thereto or thereby, but is susceptible of many other uses andadaptions, as well as changes in form and detail within the scope of the appended claims.

I claim:

1. A brake drum comprising a hollow cylindrical body of ferrous metal having an inner cylindrical surface adapted to serve as the friction surface of the brake and an outer cylindrical surface, a casting of a light metal of the class consisting of aluminum and aluminum base alloys overlying the outer cylindrical surface of said body, said casting having an annular portion encircling said body and circumferential heat dissipating fins extending in substantially parallel relation outwardly from said annular portion and being joined at their inner edges by said annular portion of the casting, said annular portion having a thickness of about one-tenth of an inch, and a film of ferro-aluminum alloy bonding the abutting surfaces of the body and said annular portion together, the thickness of said annular portion of said casting being less than that producing a reversal of stresses when heated to a temperature exceeding 250 F.

2. A method of making a composite article comprising bringing a substantially rigid ferrous metal article into contact with a molten aluminum base metal to form a film of ferro-alurninum alloy on a surface of said article, casting on the alloy film a layer of aluminum base metal and forming the layer to a thickness (t) approximating and not materially exceeding that determined by the formula wherein K is about 1.165 x on is the coefiicient of thermal expansion of the said aluminum base metal, T is a maximum operating temperature of the article in excess of 400 F. and T; is a stress reversal temperature of the i= cotan 6 composite structure of the two metals constituting the article between about 250 F. and 400 F.

3. A composite brake drum which is subjected to temperatures between about 400 F. and 1100 F., in its field of use, said article being composed of an inner hollow cylindrical drum member formed of ferrous metal and a heat dissipating casting of an aluminum base metal having an inner annular shell encircling said drum member and a plurality of circumferential heat radiating fins projecting from and integral with said shell, said drum member and casting being bonded together by an alloy predominantly of the two metals, said annular shell having a thickness not exceeding a limit of about 0.3 inch when the article is to be subjected to a temperature not exceeding about 400 F and not exceeding about a limit of 0.1 inch when the article is to be subjected to temperatures as high as 1,000 E, the thickness of the layer being selectively variable between said limits substantially uniformly in inverse relation to variations in said temperature between about 400 F. and 1,000 F. and not substantially less than 0.1 of an inch.

References Cited in the file of this patent UNITED STATES PATENTS 1,720,853 Norton July 16, 1929 1,966,130 Norton July 10, 1934 1,984,134 Himmel Dec. 11, 1934 2,008,040 Roseberry July 16, 1935 2,082,622 Fink June 1, 1937 2,396,730 Whitfield Mar. 19, 1946 2,401,235 Farr May 28, 1946 

1. A BRAKE DRUM COMPRISING A HOLLOW CYLINDRICAL BODY OF FERROUS METAL HAVING AN INNER CYLINDRICAL SURFACE ADAPTED TO SERVE AS THE FRICTION SURFACE OF THE BRAKE AND AN OUTER CYLINDRICAL SURFACE, A CASTING OF A LIGHT METAL OF THE CLASS CONSISTING OF ALUMINUM AND ALUMINUM BASE ALLOYS OVERLYING THE OUTER CYLINDRICAL SURFACE OF SAID BODY, SAID CASTING HAVING AN ANNULAR PORTION ENCIRCLING SAID BODY AND CIRCUMFERENTIAL HEAT DISSIPATING FINS EXTENDING IN SUBSTANTIALLY PARALLEL RELATION OUTWARDLY FROM SAID ANNULAR PORTION AND BEING JOINED AT THEIR INNER EDGES BY SAID ANNULAR PORTION OF THE CASTING, SAID ANNULAR PORTION HAVING A THICKNESS OF ABOUT ONE-TENTH OF AN INCH, AND A FILM OF FERRO-ALUMINUM ALLOY BONDING THE ABUTTING SURFACES OF THE BODY AND SAID ANNULAR PORTION TOGETHER, 